Ingenious software enables operators to move the most energy-consuming processes of the wastewater system to times of the day with plenty of cheap renewable energy. In this way, the plants help to stabilize the energy system.

The wastewater treatment plants create a better aquatic environment, but why not also let them lend the climate a helping hand? The plants can—more specifically—support renewable energy by placing the power consumption at times with plenty of wind energy in particular.

This is the starting point of an industrial research project that brings together DTU’s knowledge of control methods with Krüger’s experience within wastewater treatment plant management. Krüger was one of the pioneers when Denmark began to remove nutrients from wastewater in the 1980s, and has since delivered a wide range of wastewater treatment plants.

Lower electricity bill

In Kolding—Denmark’s 7th largest city—software from Krüger controls both Kolding’s main wastewater treatment plant and the drainage system.

In the industrial research project, the company uses so-called Model Predictive Control (MPC) to control parts of the system. The method combines a mathematical process model with ongoing measurements of the wastewater flow as well as electricity price forecasts.

The mathematical model is used to predict what will happen in future, thus allowing the software to control the plant so that it uses a lot of power when the electricity price is low, for example at night, or when there is plenty of wind power.

“We have demonstrated that it’s possible to control the processes by means of MPC,” says Industrial Postdoc Rasmus Halvgaard, who divides his time between Krüger and DTU Compute. “How much we can cut the plant’s electricity is not entirely clear yet, but we are undoubtedly looking at double-digit percentage savings.

Software capable of predicting the electricity price can be used to intelligently control the operation of the the wastewater treatment plants, so that the energy-demanding processes take place when the electricity price is low. In this way, the wastewater treatment plants can serve as a kind of energy storage facilities and help stabilize the energy system.

Supporting Danish exports

Already in the 1990s, Krüger introduced control software, which minimizes the plants’ energy consumption. The new project is a natural extension of that work,” says Innovation Manager Morten Grum from Krüger:

“Incorporating the latest knowledge of process management in our products is one of the ways in which we differentiate ourselves from our competitors in the international market. In this way, the project helps to support our exports and create more jobs in Denmark.”

A lot of research has gone into MPC in recent years. Companies in the chemical industry were the first to control their processes in this way. But also the wastewater treatment plants can see a huge potential,” says Rasmus Halvgaard:

“Water flows at a relatively low speed. This means that we have enough time to perform the necessary calculations along the way.”

The industrial researcher himself has written the algorithms which controls the system—in the open source programming language and the analysis tool R.

“Krüger uses, among other things, flexible control software that can communicate with R, which means that I can use many advanced R functions and don't have to reinvent the wheel. At the same time, I benefit from Krüger’s remote access to the plant. It’s a huge advantage that I can test new control algorithms online and instantly test whether they work as intended.”

Control based on weather forecasts

The project is favoured by the design of the electricity system. Large electricity consumers such as wastewater treatment plants get the full benefit of using power when it is cheap.

“Moving your power consumption according to the hourly electricity rate is thus a low-hanging fruit,” says Rasmus Halvgaard.

Each day at 12.00 noon, the European energy exchange Nord Pool Spot announces the price—hour by hour—for the coming day.

“We have achieved a large cost reduction just by rescheduling when the water is pumped into the drainage system,” explains Rasmus Halvgaard.

The majority of the savings on the electricity consumption in the project is attributable to this very simple change.

The partnership between Krüger, DTU, and Innovation Fund Denmark also includes an industrial PhD project, which is carried out by Vianney Courdent, DTU Environment.

“Vianney Courdent is, among other things, studying how to predict the flow of water to the plant by means of weather forecasts,” explains Morten Grum.

“We’re very interested in being able to move, in particular, the aeration of the wastewater—a very power-consuming process—to times of the day with cheaper electricity prices. As the plant itself doesn’t have the capacity to store large quantities of wastewater, you need to include the drainage system. However, this requires that you have studied the weather forecast, as it would be very unfortunate if moving the time of aeration resulted in contamination or flooding of the system.”

Wastewater a resource

It is still too early to say how much can be won by optimizing the management of the very power-consuming processes at the plants, says the innovation manager:

“Unfortunately, there is a huge difference between how the rate systems are designed in different countries. It therefore differs from country to country how to control the plants to achieve the best possible operating economy. This also makes it difficult to say exactly how much MPC can reduce the electricity bill. On the other hand, it’s certain that this type of control helps to support renewable energy and brings wastewater treatment closer to becoming carbon neutral.”

Morten Grum is in no doubt about the long-term goal:

“We’re no longer just seeing the plants as energy-consumers. The organic substance that is removed from the wastewater is a resource which can be converted into energy. By using electricity at times with plenty of renewable energy and producing energy at times when there is a shortage of energy in the system, the wastewater treatment plants can make a positive contribution to the energy system.”

Technical University of Denmark

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